Perforating systems used for completing an oil or gas well are well known in the art. Well bores, which are drilled through earth formations for extracting hydrocarbons in the form of oil and gas, are conventionally lined by inserting a steel casing or liner into the well, and cementing at least a portion of the casing or liner in place to prevent migration of high pressure fluids up the well bore outside the casing or liner. The subterranean formation or formations having the potential to produce hydrocarbons are directly linked with the interior of the casing or liner by making holes, referred to as perforations, through the wall thereof, through surrounding cement and into the formation. Perforations are conventionally made by detonating explosive shaped charges disposed inside the casing at a location adjacent to the formation which is to produce the oil or gas. The shaped charges are configured to direct the energy of an explosive detonation in a focused, narrow pattern, called a “jet,” to create the holes in the casing.
Conventionally, well perforation systems include a firing head and a perforating gun, both of which are suspended from, and lowered into, a well on a conveyance device such as a tubular string which may comprise so-called “coiled tubing.” Well perforation systems also conventionally comprise various components including, for example, a packer, a firing pin, an explosive booster, and a time delay device. A time delay device is needed to provide an operator sufficient time between a pressurizing event and a subsequent perforation event in order to pressure balance a well for perforation to secure optimal flow of oil or gas flow into the well. Pressure balancing a well is an important procedure because failure to do so, or if the procedure is done incorrectly, may lead to equipment damage as well as possible injury to equipment operators if insufficient hydrostatic pressure is present in the casing or liner or, if too great a hydrostatic pressure is present, the producing formation exposed by the perforating operation may be contaminated or production compromised or prevented without remedial measures. Additionally, with a properly pressure-balanced well, producing formation fluid will immediately and rapidly flow upward through the interior of the tubular string and toward the earth's surface in an appropriate, controlled manner. Therefore, it is important that the timing delay device employed be reliable and accurate in order to allow for adequate time to pressure balance a well. Time delay devices currently used in the art employ pyrotechnic time delay fuses. As described below in greater detail, pyrotechnic fuse-based time delay devices have reliability and accuracy concerns, as well as time limitations which may eventually lead to greater complexity and increased costs for customers of the oil tool industry.
FIG. 1 illustrates a conventional well perforating system 20 within well 10. The well 10 is constructed by first drilling a well bore 12, within which a well casing 14 is placed and cemented in place as indicated at 16. The perforating gun 34, mechanical release 28, packer 24, and firing head 32 are, among other components, carried by tubular string 22. The perforating gun 34 and firing head 32 are lowered on the tubular string 22 to a selected location in the well 10 adjacent to the subsurface formation 18 which is to be produced. A seal is provided by packer 24 between the exterior of tubular string 22 and wall 38 of casing 14 to define a well annulus 40 above packer 24 and an isolated zone 42 below packer 24. Perforating system 20 also includes a vent 56 located below packer 24. Vent 56 allows for a direct link between the isolated zone 42 and tubing bore 58 to ensure fluid pressure within tubing bore 58 and isolated zone 42 are substantially equal. At the time designated to fire the perforating gun 34, an actuating piston 50 within firing head 32, is moved in response to an increase in fluid pressure in tubular string 22 initiated by the operator. The movement of the piston 50 releases a firing pin 52, thus initiating a firing sequence.
As mentioned above, conventional perforating systems may provide for a pyrotechnic time delay device 30 located within firing head 32. The pyrotechnic time delay device 30 provides for a time delay between the initiation of the firing head 32 and the subsequent firing of the shaped charges carried by the perforating gun 34 in order to, as described above, pressure balance the well 10 for optimal perforation. Pyrotechnic time delay devices as known in the art provide a maximum time delay of eight minutes. Therefore, in order to achieve longer delays, an operator is forced to string multiple pyrotechnic time delay devices together in a series formation. For example, additional delays may be coupled together so as to achieve a longer delay timer.
Due to the time and expense involved in perforating well bores and the explosive power of the devices used, it is essential that their operation be reliable and precise. Stringing together multiple pyrotechnic time delay devices diminishes the system's reliability and increases the system cost and complexity.
There is a need for methods and apparatuses to provide increased system reliability and flexibility of operation of well perforating systems. Specifically, there is a need for a time delay device used in a well perforating system to allow for adequate and precise timing of operation of a well perforating system in order to pressure balance a well for optimal perforation results. Such a time delay device would desirably exhibit a high level of reliability at a low level of cost and complexity of fabrication.